Already known is a so-called secondary air injection apparatus for decreasing HC (hydro carbon) and CO (carbon monoxide) component emissions from an internal combustion engine. In such an apparatus air from an air pump driven by a crankshaft of the engine is introduced into an exhaust manifold of the engine through an air injection system including an air injection pipe, a check valve and air injection manifold. The secondary air thus introduced into the exhaust manifold operates to oxidize the CO and HC components remaining in the exhaust gas for suppressing toxic emission from the engine.
If the engine has a catalytic converter arranged in the exhaust system, it is necessary to decrease the amount of the secondary air introduced into the exhaust system during high speed and heavy load engine operation in order to prevent the catalytic converter from being overheated. Therefore, in a known air injection apparatus for an internal combustion engine provided with a catalytic converter, a pressure relief valve is connected to an output of the air pump of the apparatus. The relief valve has a spring-urged valve member adapted for discharging an extra amount of air from the air pump into the atmosphere, when the pressure in the air injection pipe is increased. However, with this type of air injection apparatus the amount of air can not be fully and effectively decreased during the high rotational speed operation, because it is not possible to maintain a predetermined constant pressure of secondary air merely by a pressure relief valve. Therefore, overheating of the catalytic converter during high rotational speed operation can not be effectively prevented.